1. Field of the Invention
The present invention relates to microlithography, and more particularly, to illumination systems for microlithographic equipment that have high numerical apertures.
2. Related Art
Photolithography (also called microlithography) is used for manufacturing of semiconductor devices. Photolithography uses electromagnetic radiation, such as ultraviolet (UV), deep UV or visible light to generate fine patterns in a semiconductor device design. Many types of semiconductor devices, such as diodes, transistors, and integrated circuits, can be fabricated using photolithographic techniques. Exposure systems or tools are used to implement photolithographic techniques, such as etching, in semiconductor fabrication. An exposure system typically includes an illumination system, a reticle (also called a mask) containing a circuit pattern, a projection system, and a wafer alignment stage for aligning a photosensitive resist-covered semiconductor wafer. The illumination system illuminates a region of the reticle with a preferably rectangular slot illumination field. The projection system projects an image of the illuminated region of the reticle circuit pattern onto the wafer.
As semiconductor device manufacturing technology advances, there are ever increasing demands on each component of the photolithography system used to manufacture the semiconductor device. This includes the illumination system used to illuminate the reticle. For example, there is a need to illuminate the reticle with an illumination field having uniform irradiance. In step-and-scan photolithography, there is also a need to vary a size of the illumination field so that the size of the illumination field can be tailored to different applications and semiconductor die dimensions.
Some illumination systems include an array or diffractive scattering optical element positioned before the reticle. The scattering optical element produces a desired angular light distribution that is subsequently imaged or relayed to the reticle.
Additionally, commonly-used die dimensions are 26×5 mm, 17×5 mm, and 11×5 mm. Thus, a standard zoom lens needs to accommodate variation in the size of the illumination field. However, a particular problem arises in the field of microlithography, where different features that are required to be formed on the semiconductor substrate require variable partial coherence on the part of the exposure optics. Specifically, partial coherence (σ), which in microlithography is commonly defined as the ratio of a numerical aperture of the illumination optics and a numerical aperture of the projection system, needs to vary depending on the nature of the feature being formed on the semiconductor substrate, e.g., the σ for trench formation may be different from the σ for line formation.
Accordingly, a need exists for a simple microlithographic system that can vary the partial coherence parameter over a large range, while simultaneously being able to accommodate different field sizes.